The present disclosure relates to the production of polymer foam sheets, planks, and the like. More particularly, the present disclosure relates to a process for converting cross-linked, closed-cell polymer (e.g., polyethylene or ethylene-vinyl acetate (EVA)) foam materials into a wide range of engineered sheet or plank products of indefinite length, and to an apparatus for carrying out such a process. The starting foam materials may be derived from virgin or postindustrial waste sources, or from a combination of both. Greater cost efficiencies are achieved when a greater percentage of the starting material is obtained from waste sources.
It is known to shred, grind, or otherwise comminute cross-linked closed-cell foam starting materials into particulate form, and to thereafter heat fuse or flame laminate the particles together under pressure to form laminated sheet or plank products. Two basic types of technologies are known for producing such sheet or plank products from cross-linked, closed-cell polyethylene or ethylene-vinyl acetate (EVA) foam starting materials. The first of these prior art technologies, commonly known as the “chimney” process, is exemplified by the process of U.S. Pat. No. 4,417,932 (Breitscheidel et al.), the disclosure of which is hereby incorporated by reference. The Breitscheidel and other similar chimney processes introduce the comminuted foam particles by gravity into a hot air chamber or “chimney”, where the particles or granules are exposed to temperatures in the range of between about 100° C. and about 200° C. as they fall by gravity onto a moving bottom conveyor, where they fuse with one another. This bottom conveyor carries the particles accumulating at the bottom of the chimney toward a secondary top conveyor that compacts the fused particles between both conveyors into a sheet-like layer of target thickness prior to cooling by water and/or air.
The limitations of such chimney processes include the following, without limitation: i) it is difficult to control dosing or the amount of foam particles introduced into the chimney heating chamber to achieve and maintain a uniform end product; ii) gravity feeding produces uneven exposure to heat (heavier particles fall faster, having shorter heating or “dwell” time), thus causing inconsistent quality and strength of the finished product; iii) light particles under the same constant temperature spend more time in the heating chamber (longer dwell time) and are thus are over-exposed to temperature which, in turn, overheats the lighter particles causing blistering or complete deterioration thereof, with the result being partial or no fusion and inconsistent quality of the end product; iv) gravity feeding into the chimney is confined to the use of foam particles having approximately the same density, weight and/or dimensions to maintain uniformity of finished product, which limits the use of chimney heating technology to foams having equal particle weight, thickness, and size; v) because chimney heating is based on temperature and dwell time exposure, and because the density, specific gravity, and fusion temperature of cross-linked, closed-cell foam starting materials vary significantly from one foam to another, it is therefore not possible to use the conventional chimney technology for a broad range of foam staring materials, combined or otherwise; and finally vi) the nature of the chimney technology also has inherent difficulty in providing an even distribution of fused particles. Once the particles are fused at the bottom of the chimney, it is extremely difficult to produce an even thickness or density on the sheet-forming conveyor, which results in end products of inconsistent quality and limited end use applications. Thus, because chimney heat-fusion technology is confined to specific foam starting materials having uniform particle size, thickness, weight and density, and because the finished products of this technology lack product consistency in terms of dimensional tolerances and product density, it has therefore experienced limited market acceptance.
The second type of prior art technology known for producing laminated sheet or plank products from cross-linked, closed-cell foam starting materials is known as “press batch” type technology. This technology is a batch process operation, which produces a foam sheet or plank that is limited in its dimensions to the size of the press bed, the female mold portion resting thereon, and the platen used as the (male) mold closure. More particularly, in this process, the comminuted cross-linked foam particles are dispensed manually into a cavity of the female mold portion. The press platen is then lowered to close the mold cavity (pressure being optional, subject to the desired finished product), and the necessary heat is transferred by conduction from the heated platen and/or from the female mold into the foam workpiece to form the end product.
Shortcomings of the press batch type of technology include the following, without limitation: i) the process is limited to producing foam sheets or planks one at a time (i.e., it is not a continuous process; ii) the product is limited to the dimensions of the female mould cavity and the cooperating press platen; iii) the foam starting material used must be shredded or otherwise comminuted to a size ranging from about 1″ (25 mm) to about 2″ (50 mm), with a thickness greater than about ¼″ (6 mm), in order to promote adequate bonding between the particles of the resulting sheet or plank; and iv) the thickness of the sheets or planks produced is limited, because there is a limit to how much sheet thickness can be heated by convective heating into the interior of the sheet. The outer surfaces of the sheet tend to be heated to a higher temperature than the interior of the sheet, thereby resulting in non-uniform bonding between the foam particles through the sheet thickness. Thus, the press batch technology is limited to a relatively narrow range of foam starting materials and a relatively thin end product, and is relatively expensive because of its time-consuming batch nature and the use of relatively expensive, close-tolerance molds. Accordingly, press batch type technology is of limited application, and is not cost-effective in the marketplace, particularly where high-volume, large-sized end products are required.